Bonzer bottom? I remember single fin days not long before the thruster when we were trying the step bottom about 2 ft in front of the fin. The idea to create a void around the back of the board to reduce friction and loosen it up. Had one for a couple of years but can't remember how well it worked, if at all. The step was about 1/4 inch down. Similar principle to the golf ball dimples but a larger scale.

Back in the "Olden Days" 1989. a young Liam, or was it Garrett McNamara had dimples all over (mostly the back half) of his board to give him extra speed for his aerial launching. At the moment it really is looking like the 80' was the golden age of fashion and that includes surfing. Shorter shorts, beards, chicky boards and now dimples.
How lucky I was as a kid.

A mate that rides a ski has 2 holes a few inches forward of the front fins (thruster setup) that go all the way to the deck. Apparently it creates some air bubbles to stop the ski's large concave from sticking to the water too much and tracking. Seems to work he really can throw this thing around (airs and fins free reos) and it is really fast.